Sunscreens are cosmetic compositions which are applied topically to human skin to provide protection against the harmful ultraviolet rays of the sun (UV-A and UV-B radiation, generally in the range of 290-400 nm). Conventional sunscreens are prepared using cosmetically acceptable lotions, oils, creams, and emulsions (both oil-in-water and water-in-oil). Typically, organic agents have been employed in sunscreen compositions as the active ingredient. For example, PABAs (p-aminobenzoic acids), benzophenones, salicylate esters and dioxybenzone have been used. More recently, inorganic agents have been employed in sunscreen compositions. For example, zinc oxide, titanium dioxide, and calamine.
The inorganic agents, such as titanium dioxide, have a tendency to agglomerate which reduces their efficacy as UV screening agents in cosmetic sunscreen compositions. Additionally, sunscreen compositions containing titanium dioxide have an unpleasant feel on the skin of the user, particularly when higher concentrations of titanium dioxide are used. Further, the titanium dioxide sunscreens usually contain levels of the active ingredient which result in increasing whiteness/blueness on the skin, i.e., increased visible light opacity. Therefore, the cosmetic formulator is confronted with several problems when formulating sunscreen compositions employing titanium dioxide.
In general, formulation optimization, always a goal of the cosmetic formulator, for sunscreens (and sunblocks) aims for higher SPFs (Sun Protection Factor), mildness and water-resistancy. Ken Klein, DCI pp. 22-30, August 1989. Cosmetic formulators have had the option of using titanium dioxide as the active sunscreen ingredient since early 1978 when it was approved by the OTC Panel of the FDA for use in sunscreen preparations. Nevertheless, the problems mentioned above have persisted.
The grade of titanium dioxide used in cosmetics, particularly as sunscreens, is different than that used as a pigment in the paint, paper and plastics industries. The most distinct difference is that of particle size. The particle size of titanium dioxide used as a pigment is such that its use in cosmetics is limited by the increase in whiteness on the skin.
Schwartz and Peck reported in 1947 that heavily pigmented preparations (liquids, creams or powders) will prevent or reduce the passage of the UV radiation, but while preventing sunburn, such preparations will also prevent suntan. Zinc oxide, calamine and titanium dioxide are the most effective in this regard. Cosmetics & Dermatitis 1947, Paul B. Hocker, New York, 145. Again, an ideal sunscreen requires the ability to attenuate both UVA and UVB radiation, while being chemically inert, non-toxic, non-irritating and non-sensitizing.
Brown and Galley have reported that the appropriate UV opacity can be maintained by titanium dioxide particle sizes as small as 100 nm and less (microfine or ultra-fine) while reducing visible light opacity, thus making titanium dioxide cosmetically acceptable. Cosmetics & Toiletries 105: 69-73 (December 1990). They also reported that titanium dioxide particle sizes, selected to have acceptable cosmetic properties, can still exhibit broad spectrum activity, offering protection to both UVB and UVA wavelengths, unlike organic sunscreens which are generally either UVA or UVB specific. Id.
Clearly, there was a breakthrough in cosmetic technology when it was discovered that if titanium dioxide was produced in an ultra-fine form (microfine), it was possible to obtain transparent films on the skin. However, the degree of protection conferred to the user would depend on the concentration of the ultra-fine titanium dioxide in the sunscreen preparation. Generally, to achieve high SPF (Sun Protection Factor) values, higher concentrations of titanium dioxide are needed which returns one to the problem of whitening color on the skin and unpleasant or grimy feels. (See Diffey and Robson, J. Soc. Cosm. Chem., 40: 127-133 (May-June 1989) for a system for testing SPF's both in UVA and UVB regions.) Philip Alexander reports that 1% of ultra-fine titanium dioxide, as the active sunscreen agent, in a sunscreen product would give an SPF of between 2 and 3. Manufacturing Chemist 62(7): 21 (July 1991).
Titanium dioxide has been described in the literature as a brilliant white, amorphous and odorless powder. It is found in nature as the minerals rutile and anatase. Philip Alexander has reported titanium dioxide to be insoluble in water and in fatty acid esters in which it disperses. Manufacturing Chemist 62(7): 21 (July 1991).
Ultra-fine titanium dioxide is available commercially from several vendors for cosmetic use. Titanium dioxide is available with either an inorganic or organic coating. Aluminum stearate and aluminum oxide coatings are common inorganic coatings. Aluminum laurate and aluminum hydroxide are also used as coatings on titanium dioxide. In addition, Tioxide Chemicals, U.K. manufactures several grades of ultra-fine titanium dioxide which are surface treated and dispersed in a variety of organics, such as mineral oil/triglyceride, octyl palmitate and isopropyl myristate, using a dispersing agent.
Since the introduction of microfine titanium dioxide, there has been an increase in the use of titanium dioxide as a sunscreen. With the increase in experience with titanium dioxide, additional problems have arisen such as the observation that microfine titanium dioxide has a propensity to agglomerate (clump) into large particles. Several attempts have been made to remedy this and the other problems mentioned above, e.g., to produce new particle size titanium dioxide.
International Publication No. WO 90/06103, published Jun. 14, 1990 reports that coating titanium dioxide particles of a size less than 100 nm with phospholipids reduces their tendency to clump and enables the particles to be more effectively dispersed. It is reported that the enhanced dispersibility of the particles allows higher concentrations of titanium dioxide than were previously possible to be incorporated into stable fluid emulsions and dispersions. The enhanced dispersibility of the phosphorized coated particles is also alleged to improve UV screening efficiency.
U.K. Patent Application GB 2,226,018A, published Jun. 20, 1990 describes an aqueous dispersion of acicular fine particle size titanium dioxide (largest dimension is 0.01 to 0.15 microns) containing 20 to 60% by weight solids plus a polycarboxylic acid dispersant. The aqueous dispersion is milled to produce a product absorbent to ultra-violet (UV) light and substantially transparent to visible light.
U.S. Pat. No. 4,927,464 describes a form of titanium dioxide which is ocular in shape, having a ratio of the largest dimension to the shortest within the range 8:1 to 2:1 and wherein the largest dimension is from 0.01 to 0.15 microns and in which the particles have a coating of a hydrous oxide of aluminum and of silicon in a weight ratio of at least 1.5 and not greater than 4.5 expressed as the oxides.
U.K. Patent Application GB 2,206,339A describes a dispersion of titanium dioxide particles of size 0.01 to 0.15 microns employing an organic oil, such as vegetable oils, fatty alcohols, saturated fatty acid diesters and linoleic glycerides; and a dispersing agent based on one or more polyesters or salts of a hydroxycarboxylic acid and a carboxylic acid free of hydroxy groups. The U.K. patent application discloses that other suitable dispersing agents are monoesters of fatty acid alkanolamides and carboxylic acids and their salts based on 6-226 (un)saturated fatty acids. It is also reported therein that sunscreen preparations which contain these types of dispersions were prepared. The inventors of the U.K. patent application report that 12.5% w/w of the titanium dioxide dispersion (42% solids) gives a weighted cast SPF of 6; and a 25% w/w dispersion (42% solids) gives a weighted cast SPF of 7.3. Similarly, other sunscreen preparations containing 12.5% and 25% w/w of the titanium dioxide dispersion (42% solids) gave weighted cast SPFs of 9.3 and 10.8, and in vivo SPFs of 10.2 and 11.6, respectively.
As mentioned above, many cosmetic sunscreen preparations employing titanium dioxide as the active sunscreen, have been prepared. For example, U.S. Pat. No. 5,028,417 describes a sunscreen composition containing an extending medium and titanium dioxide having a particle size of less than 10 nm(m.mu.). Further, the U.S. Pat. No. 5,028,417 describes a dispersion formulation containing an 80/20 mixture of titanium dioxide with barium sulfate coated with stearic acid (62% titanium dioxide) dispersed in isooctyl stearate by means of a pearl mill (Dyno-Mill Type KDL). It is reported that the resulting dispersion contains 15.5% titanium dioxide and has an SPF of 14.6.
U.K. Patent Application GB 2,217,987A describes sunscreen compositions containing 0.5 to 30% by weight of titanium dioxide having a primary size of less than 100 nm and which is coated with aluminum stearate; 5 to 20% by weight of an oil phase; 1 to 15% by weight of an emulsifier; and at least 40% by weight of an aqueous phase.
From the above discussion, it should be apparent that inorganic sunscreen agents like ultra-fine titanium dioxide are superior to traditional organic sunscreen agents (less irritation potential and better/broader UV protection). Because of its unique properties, ultra-fine titanium dioxide is being increasingly used in cosmetic sunscreens today. Nevertheless, there are still many problems associated with the use of ultra-fine titanium dioxide. In particular, the level of ultra-fine titanium dioxide powder necessary to achieve proper (and higher) SPF levels makes the product aesthetically unacceptable, i.e., there is a heavy and draggy feel (which may also result in skin irritation) and a white/blue residual on the skin. Similarly, commercially available ultra-fine titanium dioxide has a propensity to agglomerate into clumps of much higher particle sizing. Use of such agglomerations is unacceptable for cosmetic sunscreens.
It is an object of the present invention to provide a novel dispersion of titanium dioxide, without the use of any dispersing agent, which achieves higher SPF levels with lower levels of titanium dioxide.
It is also an object of the present invention to provide cosmetic sunscreen compositions with proper SPF levels while eliminating the unacceptable heavy feel, residual color and other undesirable properties associated with certain levels of titanium dioxide.
Further, it is an object of the present invention to provide a process for dispersing ultra-fine titanium dioxide particles prior to utilization in cosmetic sunscreens.
It is yet a further object of the present invention to provide cosmetic sunscreen compositions which do not utilize organic sunscreens and have high SPF while maintaining good feel and transparency.